Many oil fields are comprised of reservoirs wherein the injection wells involved with a waterflood operation have or develop direct or indirect pressure communication with offset production wells. When this pressure communication develops, the producer well exhibits a breakthrough of injected water and thus an increase in the water oil ratio and decrease in oil production. The waterflood process is effectively broken as a result. This breakthrough of injection water is a serious disadvantage and becomes more prevalent as the oil field matures. A similar problem for a conventional waterflood of a light oil reservoir is generally described in U.S. Pat. No. 7,243,720.
As used herein, the term “viscous/heavy oil” (also simply “heavy oil” for ease of reference) means an oil of 30° API gravity or less, and generally less than 25° API. Some heavy oil reservoirs in the State of Alaska, USA or in Canada can exhibit a gravity of less than 17° API and as low as 10° API.
Though there may be communication, the injector well may still hold injection pressure suggesting that the flooding operation of the reservoir has not completely failed. Typically, the pressure difference between the bottom hole injection well pressure and the bottom hole production well pressure is on the order of 1000 to 2000 psi, and usually 1200 to 1500 psi in a normal heavy/viscous oil waterflood. When communication occurs, this pressure differential will decrease. In severe instances, such as a matrix bypass event or “MBE” (described in more detail hereinafter) frequently the waterflood will completely fail which is signified by the bottom hole injection pressure of the injector essentially equaling the bottom hole producing pressure of the producer.
A waterflood process may also be used for production of heavy/viscous oil from a reservoir combined with a cold production method wherein sand is produced with the water/oil mixture. This cold production method is sometimes referred to as a CHOPS (Cold Heavy Oil Production with Sand) method. Conventional waterflooding of heavy/viscous oil reservoirs is plagued with either very large pressure gradients or premature water breakthrough as previously mentioned, and both these aspects can lead to low injectivity or poor sweep efficiency, thereby resulting in poor oil recovery. Further, because of the increased viscosity of the heavy oil, it is susceptible to fingering. Sand production is not a major problem with production of heavy/viscous oil, since the wells are designed and operated, such as in the case of the CHOPS method, to produce sand with oil.
A particular characteristic of the CHOPS process which is a concern for waterflooding is the formation of so-called “wormholes” or channels which are formed as the fines are removed from the reservoir rock during production of the oil/sand mixture. An article by Tremblay et al. “Simulation of Cold Production in Heavy-Oil Reservoirs: Wormhole Dynamics”, SPE Reservoir Engineering (May 1997) at pages 110-117, advises that fines removal causes the permeability of the rock to increase as the wormhole develops. In addition, the matrix of the rock becomes weaker and weaker as the process continues, even to the point where it is believed that a portion of the rock formation can fail and produce large volumes of sand and leave a “void” in the reservoir. Short of this event, the enhanced permeability rock where the fines have been removed is also sometimes called the “halo”. Wormhole formation is described, for example, in U.S. Pat. No. 7,677,313, the teachings of which are incorporated herein by reference in their entirety. Wormholes may comprise either “void” spaces, “halo” regions, or both.
On the water injection side, those in the art can appreciate that so-called “fingering” of injected water through heavy oil also becomes a significant factor. The water fingering makes the waterflood process very inefficient.
When both producer and injector wells are active in a heavy/viscous oil water flood, it is believed, without wishing to be bound by theory, that a wormhole from the producer side seeks the relatively high pressure source of the injector well and correspondingly a water finger from the injector side seeks the lower pressure of the producer well. When this finger of water connects to the wormhole of the producer, the water-oil ratio of the produced fluids increases dramatically and there becomes a pressure communication between the injector and the producer.
Matrix Bypass Events (MBE's) as mentioned above are a particular problem in the waterflooding of many heavy/viscous oil reservoirs which use a cold production method, such as a CHOPS process. Basically the water injector well develops direct pressure communication with the oil production well. The presence of an MBE can be inferred in most cases where the difference in pressure between the bottom hole injection well pressure and the bottom hole production well pressure (ΔPbh) shows a significant decrease over a relatively short period of time, such as a change in ΔPbh on the order of at least 100 psi over a 12 hour time period. In the most severe cases, the pressure differential (ΔPbh) can be less than 200 psi and even less than 100 psi, i.e., the bottom hole injection pressure of the water injector approaches and nearly equals the bottom hole production pressure of the producer. This short circuit of the injected water can make the waterflood ineffective and oil recovery not economically feasible, since the injector and producer are just primarily cycling water.
U.S. Pat. No. 7,677,313, previously incorporated herein by reference, discloses a method for controlling water influx into wellbores by blocking high-permeability channels. The method involves use of both an unreinforced and reinforced gel which is introduced into the channel to block the flow of water therethrough. U.S. Pat. No. 6,720,292 discloses the use of a reinforced clay gel to control fluid flow, while U.S. Pat. No. 7,350,572 discloses use of a mixture of a treatment fluid (gelled acid material) and a fiber to control fluid losses from a wellbore.
While some of these methods may block fluid flow, the materials employed may not provide a complete seal, or any blockage may only be temporary, resulting in a relatively rapid reoccurrence of fluid influx.
What is needed is a method which may be used to remediate or repair a zone, such as that associated with wormholes wherein a void space and associated halo region are created due to production of sand, as in the case of a CHOPS type cold production process, and have resulted in an undesired pressure communication between the production well and an associated drive fluid injection well. Such a method could be used to control fluid flow and restore the effectiveness of the associated flooding operation and increase the amount of hydrocarbons produced.